The invention relates to the analysis of frequency spectra of a frequency modulated continuous wave (xe2x80x9cFMCWxe2x80x9d) microwave radar system, in which there is at least one virtually constant interference frequency.
In FMCW radar systems which are used, for example, as distance sensors, the parameters of the range and speed of a measurement object are obtained by analysis of the frequencies contained in a radar signal. All known frequency analysis methods (for example, that described in WO99/10757) have limited resolution, however, by virtue of the system. This creates a critical difficulty in determining a frequency in the immediate vicinity of another frequency.
FMCW signals often contain not only the desired useful frequencies fN which are caused by reflections on measurement objects in the measurement area, but also undesirable systematic interference components at virtually constant frequencies fS, which create a critical difficulty in determining fN in their immediate vicinity. It is therefore necessary to attempt to suppress these interference components effectively.
A method for suppressing systematic interference frequencies is described in S. V. Vaseghi: xe2x80x9cAdvanced Signal Processing and Digital Noise Suppressionxe2x80x9d, Wiley Teubner, Chichester 1994. This method assumes that the frequencies, amplitudes and phases of the interference components vary from one measurement to the next. DE 43 32 071 A1 describes a method in which the interference signal reflected on the antenna is compared with a previously recorded signal profile, in order to detect offsets occurring at the antenna. This method assumes that the frequencies, amplitudes and phases of the interference components vary only due to external influences, such as antenna offsets.
The interference components in FMCW signals may be caused by internal reflections in the electronics and on the antenna, or by external reflections, for example on the bottoms of containers, and on container struts. Irrespective of the particular measurement process, they actually have virtually constant, a-priori known frequencies fs, which can either be measured or can be calculated from the geometry of the radar sensor. The associated amplitudes and phases, in contrast, fluctuate severely, for example due to drift in the radar electronics, which changes the radar mid-frequency, and are therefore a-priori unknown.
One potential object of the present invention is to specify a method for evaluation of a measurement signal of an FMCW radar system by which systematic interference due to fixed-position reflectors, which cause internal or external reflections, can be effectively eliminated.
The method according is based on evaluation of the radar signal by frequency spectrum analysis. The interference frequencies which are present in the spectrum and result from interference reflections are eliminated by specific mathematical methods. For this purpose, a reference measurement is first of all carried out which, depending on the nature of the interference frequencies to be eliminated, is carried out without or with only one measurement object as the reflection target. The frequency spectrum resulting from the reference measurement is preferably analyzed with a greater frequency resolution than is used in normal measurement operation of the apparatus. The frequencies of the interference components in the radar signal are determined in this way. The method may be based on an adaptive least-squares-fit determination, matched to the individual measurement signal, of the interference amplitudes and phases for the known interference frequencies. The interference signal, adapted from one measurement to the next, is thus calculated and is subtracted from the FMCW measurement signal. The FMCW measurement signal that has been cleaned up in this way is then subjected to one of the known frequency analysis methods. Another possibility for determining the interference components is to reconstruct them mathematically, on the basis of the system design.
Discrete values relating to the measurement signals are determined at a predetermined number of sampling points, which are offset by the same time interval with respect to one another. The values detected in this way are recorded as complex-value superimpositions of oscillation functions at the interference and useful frequencies, and have the interference components removed from them by mathematical computation methods. The measured values that have been cleaned up in this way are then subjected to one of the known frequency analysis methods.